This invention relates in general to industrial type circuit breaker testers and particularly to portable testers for on site testing of circuit breakers. In addition to being factory tested, power circuit breakers are generally operationally tested in the field under low voltage conditions to verify their capability for tripping rated fault currents in a prescribed time. Two types of testing, generally categorized as unpowered or "dry" testing and primary injecting testing, are used. These tests require technicians to bring expensive and cumbersome test equipment to a construction site, which equipment generally is powered from an AC line that may not be conveniently available at the site. Regulations that govern installation of breakers in distribution systems also require system testing to verify that the wiring is correct and that the circuit breakers operate properly before power is applied.
The dry test and the low voltage high current test (primary injection) provide a relatively simple test sequence for determining the operability of the electronic tripping circuitry of the breaker. This circuitry is contained in a structure referred to as a trip unit. Dry testing rapidly verifies the basic electronic functioning of the trip unit and subjects the breaker to simulated phase (.0.) overload and simulated ground fault (GF) tests to verify operation of any time delay circuits. Since the breaker has been tested at the factory, on site dry testing assures that the breaker electronics were not damaged during shipment or handling. Primary injection testing verifies that the wiring outside of the circuit breaker is proper as well as assuring the basic functioning of the circuit breaker before full voltage is applied to the distribution system.
In primary injection testing, a low voltage source is used to supply very high current through one (or more) poles of a breaker. Each pole is subjected to individual testing and if the breaker is equipped with integral ground fault protection circuitry, that must be defeated since the ground fault sensing mechanism will trip the breaker before the rated test current is attained. Conventionally, the ground fault sensing system is offset by attaching heavy cables to pass an equal and oppositely directed current through another pole of the breaker. This prevents detection of an unbalanced current condition by the ground fault system.
Three phase, four wire systems requiring a neutral current transformer (known as fourth CT) may require an additional power source with its attendant additional wiring. Further, the electronic trip units of the breakers may include memory circuits for integrating previous phase and ground fault currents experienced in the system. The memory circuits have time constants which may require as much as five minutes for phase tests and 30 seconds for GF tests to discharge or clear. This significantly slows down testing since the memories must be cleared after each test.
The tester of the invention is described in conjunction with Square D Company type ME/NE/PE Micrologic (Trademark) circuit breakers. These circuit breakers have a removable rating plug that is interconnected with the circuit breaker trip unit by means of a 20-pin connector. The current rating of the breaker may be readily changed by using a different rating plug, which consists of a plurality of resistive elements that are connected by the connector terminals to the various sensing circuits in the trip unit.
Overload tripping is a relatively long time test with the test current being slowly brought up to determine a so-called pickup level. At the pickup level, the delay time is measured. The delay time is how long it takes the breaker to trip after the pickup level with a continuing current overload. A five minute waiting period is required to allow the overload delay memory to clear (reset) before testing the next pole of the breaker. If the trip unit includes an integral ground fault circuit, the test current must be routed to pass in the opposite direction through another pole. There are also short time trip tests which utilize a memory for recording the effects of successive overcurrents. This memory must be discharged for 30 seconds before a subsequent test is undertaken. As mentioned, ground fault testing of three phase, four wire systems requires a separate power supply. The field test unit of the invention eliminates the need for a second power supply for powering the trip unit in the breaker.
The Square D Company Micrologic (Trademark) line of circuit breakers also include "restraint" features which permit downstream circuit breakers to restrain or hold off tripping of upstream circuit breakers. The feature results in a downstream breaker (i.e. one closer to the fault) tripping first and thereby avoiding a complete system shutdown when a fault occurs. The restraint feature must be defeated during verification testing of the short time tripping and ground fault tripping functions. In accordance with the invention, the Restraint In and Restraint Out terminals on the connector of the trip unit are shorted together, thus disabling the restraint operation.
The field test unit constructed in accordance with the invention can be used to: directly verify phase and ground fault tripping; power the trip unit and eliminate the need for an extra power supply during primary injection testing and fourth CT ground fault testing; and disable the ground fault function during phase testing.